This invention relates generally to the sealing of paperboard cartons, and more particularly to an automatic carton sealing system in which sealing of the carton flaps is effected by direct application of a gaseous flame to the surfaces thereof.
In modern packaging techniques, the container for the goods usually takes the form of a carton made of paperboard stock coated on both faces with a leakproof polyethylene layer. A carton of this type is initially blanked from coated paperboard stock having score lines to define the sides and flaps of the carton. The blank is folded and the sides are joined to create, when the blank is erected, an open-ended tube having a rectangular cross-section. The open ends of the box are provided with a pair of relatively small flaps referred to as dust flaps, and a pair of large flaps referred to as the upper major flap and the lower major flap.
In packaging goods in such cartons by automatic machinery, the usual procedure is to insert a product into the open-ended carton and to then fold in and seal the end flaps. In one such machine, after the product is inserted the dust flaps at both ends are first folded in and then the lower major flap is folded thereover, the upper edge of the lower flap being received and retained in the fold of the upper major flap which hinges outwardly.
To complete the package, it is then necessary to seal the upper major flap to the lower major flap of the carton. Heretofore, this was accomplished by blowing heated air through a triangular nozzle into the triangular shed defined by the unsealed major flaps to momentarily soften their polyethylene layers, the carton then being conveyed through compression rolls acting to press the upper major flap against the lower major flap to effect a bond therebetween upon cooling and hardening of the overlapping layers, which takes place during compression.
The use of heated air to effect sealing has a number of serious practical disadvantages. Because the heated air is introduced at high velocity from a fixed station into the triangular shed while the carton is being conveyed, unless the temperature of the air is high, it will not produce adequate softening of the thermoplastic layer to provide an effective bond. Since the heated air is introduced through a triangular nozzle that is coupled by an extended line or duct to a blower and heater assembly, the relatively small cross-sectional area of the nozzle produces an excessive pressure drop. And since the heated air is blown into an open shed to impinge on the flap layers, the air is deflected and discharged from the ends of the shed so that the heat exchange period between the air and the layers is quite brief.
The heated-air technique for sealing the flaps of cartons coated with a polyethylene layer has the advantage of being glueless, since it obviates the need to provide strips of glue on the interface of the flaps to be joined and to thereafter heat-soften the strips and press the overlapped flaps together to form a permanent bond.
But because the heated-air technique is characterized by poor heat exchange efficiency, sealing of the flaps may be incomplete or defective unless the thermoplastic layers on the flaps are subjected to heat for a sufficient period to adequatley heat the material above the softening point. Should the layers not be fully softened, the resultant joints will have inferior strength and be incapable of withstanding the usual stresses encountered by cartons in ordinary use.
In order, therefore, to insure adequate heating of the flaps by the heated-air technique, the carton must be run through the heating station at a relatively slow speed. This requirement is at the expense of the production rate which cannot be at high speed without sacrificing the quality of the joint.